A source field plate is known as an electric field relaxation technique for a field effect transistor (FET). The source field plate which is arranged between a gate and a drain is capable of relaxing concentration of electric field near a gate electrode, and as a result, an FET improves a withstanding voltage thereof. In order to realize the relaxation of the electric field effectively, the source field plate is arranged near the gate electrode, or is arranged overlapping with the gate electrode.
As for an FET which is capable of operating at high frequency, such as a millimeter wave band, a high gain is expected when it operates at relatively low frequency, such as a microwave band etc. On the other hand, however, suppression of oscillation is difficult because amount of feedback becomes large. When the source field plate is arranged, although the source field plate is effective in suppression of oscillation, the source field plate increases a capacitance Cgs between gate and source because the source field plate which is short-circuited to the source is arranged near the gate. As a result, the gain of the FET decreases.